hello-algo/codes/cpp/chapter_dynamic_programming/min_path_sum.cpp
Yudong Jin e720aa2d24
feat: Revised the book (#978)
* Sync recent changes to the revised Word.

* Revised the preface chapter

* Revised the introduction chapter

* Revised the computation complexity chapter

* Revised the chapter data structure

* Revised the chapter array and linked list

* Revised the chapter stack and queue

* Revised the chapter hashing

* Revised the chapter tree

* Revised the chapter heap

* Revised the chapter graph

* Revised the chapter searching

* Reivised the sorting chapter

* Revised the divide and conquer chapter

* Revised the chapter backtacking

* Revised the DP chapter

* Revised the greedy chapter

* Revised the appendix chapter

* Revised the preface chapter doubly

* Revised the figures
2023-12-02 06:21:34 +08:00

116 lines
3.6 KiB
C++

/**
* File: min_path_sum.cpp
* Created Time: 2023-07-10
* Author: Krahets (krahets@163.com)
*/
#include "../utils/common.hpp"
/* 最小路径和:暴力搜索 */
int minPathSumDFS(vector<vector<int>> &grid, int i, int j) {
// 若为左上角单元格,则终止搜索
if (i == 0 && j == 0) {
return grid[0][0];
}
// 若行列索引越界,则返回 +∞ 代价
if (i < 0 || j < 0) {
return INT_MAX;
}
// 计算从左上角到 (i-1, j) 和 (i, j-1) 的最小路径代价
int up = minPathSumDFS(grid, i - 1, j);
int left = minPathSumDFS(grid, i, j - 1);
// 返回从左上角到 (i, j) 的最小路径代价
return min(left, up) != INT_MAX ? min(left, up) + grid[i][j] : INT_MAX;
}
/* 最小路径和:记忆化搜索 */
int minPathSumDFSMem(vector<vector<int>> &grid, vector<vector<int>> &mem, int i, int j) {
// 若为左上角单元格,则终止搜索
if (i == 0 && j == 0) {
return grid[0][0];
}
// 若行列索引越界,则返回 +∞ 代价
if (i < 0 || j < 0) {
return INT_MAX;
}
// 若已有记录,则直接返回
if (mem[i][j] != -1) {
return mem[i][j];
}
// 左边和上边单元格的最小路径代价
int up = minPathSumDFSMem(grid, mem, i - 1, j);
int left = minPathSumDFSMem(grid, mem, i, j - 1);
// 记录并返回左上角到 (i, j) 的最小路径代价
mem[i][j] = min(left, up) != INT_MAX ? min(left, up) + grid[i][j] : INT_MAX;
return mem[i][j];
}
/* 最小路径和:动态规划 */
int minPathSumDP(vector<vector<int>> &grid) {
int n = grid.size(), m = grid[0].size();
// 初始化 dp 表
vector<vector<int>> dp(n, vector<int>(m));
dp[0][0] = grid[0][0];
// 状态转移:首行
for (int j = 1; j < m; j++) {
dp[0][j] = dp[0][j - 1] + grid[0][j];
}
// 状态转移:首列
for (int i = 1; i < n; i++) {
dp[i][0] = dp[i - 1][0] + grid[i][0];
}
// 状态转移:其余行和列
for (int i = 1; i < n; i++) {
for (int j = 1; j < m; j++) {
dp[i][j] = min(dp[i][j - 1], dp[i - 1][j]) + grid[i][j];
}
}
return dp[n - 1][m - 1];
}
/* 最小路径和:空间优化后的动态规划 */
int minPathSumDPComp(vector<vector<int>> &grid) {
int n = grid.size(), m = grid[0].size();
// 初始化 dp 表
vector<int> dp(m);
// 状态转移:首行
dp[0] = grid[0][0];
for (int j = 1; j < m; j++) {
dp[j] = dp[j - 1] + grid[0][j];
}
// 状态转移:其余行
for (int i = 1; i < n; i++) {
// 状态转移:首列
dp[0] = dp[0] + grid[i][0];
// 状态转移:其余列
for (int j = 1; j < m; j++) {
dp[j] = min(dp[j - 1], dp[j]) + grid[i][j];
}
}
return dp[m - 1];
}
/* Driver Code */
int main() {
vector<vector<int>> grid = {{1, 3, 1, 5}, {2, 2, 4, 2}, {5, 3, 2, 1}, {4, 3, 5, 2}};
int n = grid.size(), m = grid[0].size();
// 暴力搜索
int res = minPathSumDFS(grid, n - 1, m - 1);
cout << "从左上角到右下角的最小路径和为 " << res << endl;
// 记忆化搜索
vector<vector<int>> mem(n, vector<int>(m, -1));
res = minPathSumDFSMem(grid, mem, n - 1, m - 1);
cout << "从左上角到右下角的最小路径和为 " << res << endl;
// 动态规划
res = minPathSumDP(grid);
cout << "从左上角到右下角的最小路径和为 " << res << endl;
// 空间优化后的动态规划
res = minPathSumDPComp(grid);
cout << "从左上角到右下角的最小路径和为 " << res << endl;
return 0;
}